Development in the fabrication of microelectronic components tends toward an increasing integration density. In the case of DRAM (Dynamic Random Access Memory) memory chips, for example, it is prefarable to obtain an increase in storage capacity with the size of the memory chip remaining the same. Accordingly, it is desirable to increase miniaturization of structures that are to be formed in a semiconductor wafer.
A process step in the fabrication of micrometer-and nanometer-scale structures is a lithographic imaging of the structure from a mask onto the semiconductor wafer.
FIG. 2 diagrammatically illustrates an conventional apparatus for lithographic imaging in the fabrication of DRAM memory chips. A slotted illumination region 1 is provided above the mask 2, which is arranged in one plane and contains the structure to be imaged. The illumination region may, for example, have a length of 104 mm and a width of 8 mm and the rectangular mask 2 may have a length of 104 mm and a width of 132 mm. By means of the illumination region 1, a strip-type cutout on the mask 2 is illuminated and a partial structure contained in the cutout is imaged onto the semiconductor wafer 3 by means of a projection system 5, which generally de-magnifies the partial structure, by a factor of 0.25, for example. In order to illuminate, successively, the entire mask 2 and to image the structure contained in mask 2, mask 2 is moved beneath the illumination region 1 in the direction illustrated by an arrow in FIG. 2a. A movement of the semiconductor wafer 3 is coupled with the movement of mask 2. In order that the entire structure contained in mask 2 is imaged onto the semiconductor wafer 3, the semiconductor wafer 3 is moved in an opposite direction to mask 2. The opposite direction results from the interchange of right and left caused by the projection system 5. Depending on the demagnification factor of the projection system 5, the speed of the mask will be greater than that of the semiconductor wafer 3. Given a demagnification factor of 0.25, the speed at which mask 2 moves is four times greater than that of the semiconductor wafer 3.
FIG. 2a illustrates the positions of mask 2 and the semiconductor wafer 3 at the beginning of an imaging of the structure contained in mask 2. Situated above mask 2 is the illumination region 1, which is arranged above an optical axis of the projection system 5, said optical axis being indicated by the dashed line, and does not alter its position during the imaging. The mask 2 is moved in the direction indicated by the black arrow at shown in FIG. 2a during the imaging operation. At the same time, the semiconductor wafer 3 is moved in the opposite direction to the mask 2 as indicated by the black arrow at the semiconductor wafer 3. FIG. 2b illustrates the positions of mask 2 and semiconductor wafer 3 at the end of the imaging. As is evident, mask 2 has migrated from one side of the optical axis to the other side of the optical axis, while the semiconductor wafer 3 has migrated in the opposite direction. There is an air gap situated between semiconductor wafer 3 and lens surface 7.
By means of the described movement of mask and semiconductor wafer, the structure contained in the mask is scanned and progressively imaged onto the semiconductor wafer. The mask generally contains structures for one or a plurality of microelectronic components, such as memory chips for example. In the manner described above, the structure contained in the mask can be imaged once onto the semiconductor wafer. If the semiconductor wafer has a diameter of 300 mm, for example, then the structure contained in the mask can be imaged onto the semiconductor wafer approximately 150 times. In order to repeat an imaging, the semiconductor wafer, which is likewise arranged in an x-y plane, is displaced in said plane, so that, during a renewed scan operation, the partial structures are imaged onto as yet unexposed areas of the semiconductor wafer.
Step and scan systems are currently used in the production of DRAM memory chips. This system employs imaging apparatus that scan the structure contained in the mask and progressively exposing a section on the semiconductor wafer on which the structure is imaged, and, after the exposure of the section, migrate to a next section by means of a movement of the semiconductor wafer in the x-y plane and expose said next section.
The applicability of the imaging apparatus described, which effects illumination for example with a light wavelength of 193 nanometers, to ever smaller feature sizes of, for example, less than 70 nanometers can be extended with the aid of immersion lithography. During immersion lithography, a liquid is provided between the semiconductor wafer and the lens surface of the projection system that is nearest to the semiconductor wafer. The liquid completely fills the gap between the semiconductor wafer and the lens surface, thereby avoiding a light transition from lens to air. The resolution capability of the projection system is increased by avoiding the transition.
The liquid that fills the air gap, also called immersion liquid, has to satisfy a plurality of requirements. It should be transparent to the light wavelength employed and should have a predetermined refractive index. The liquid should additionally have a sufficiently low viscosity, so that sheer forces do not occur both in the event of a scan movement executed at high speed and in the event of a step movement. The term step movement denotes the movement of the semiconductor wafer that is necessary in order to bring the semiconductor wafer to a new position with regard to the projection system for a repetition of the imaging, so that the structure is projected onto an as yet unexposed section of the semiconductor wafer.
There are a number of disadvantages associated with the immersion liquid between the semiconductor wafer and the lens surface of the projection system. These disadvantages result from the scan movement of the mask and the movement of the semiconductor wafer that is coupled thereto. The movement of the semiconductor wafer relative to the immersion liquid generates turbulence and microbubbles in the immersion liquid. Such hydrodynamic effects reduce the quality of the imaging of the structure onto the semiconductor wafer. Furthermore, the immersion liquid generates a mechanical coupling between projection system and semiconductor wafer. Accordingly, a vibration in the semiconductor wafer is transmitted to the lens system, which again adversely affects the imaging quality.